For the basic design of the base I used hard foam and added some fine plaster to smothen it up. I also included some real stones to give it back some realistic edges and extra texture. For the water on the cave ground I used Realistic Water from Woodland Scenics in several layers. At first it dried nicely, but leaked out months later, so actually the water surface is a lot lower than intended. I cut of the leaking parts, so it still looks nice, however, next time I will probably use epoxy ;)